This invention relates to an electromagnetically driven actuator for an optical pickup used for recording signals to and reproducing signals from an optical recording medium such as an optical disc or an optomagnetic disc.
Conventionally, information signals are reproduced from and recorded to an information recording medium such as an optical disc, for example a so-called compact disc (CD) or an optomagnetic disc, by using an optical pickup. The optical pickup comprises a semiconductor laser as a light source, an objective lens, an optical system and a light detector.
In an optical pickup, a light beam emitted by the semiconductor laser is focused through the optical system onto a recording surface of an optical disc by the objective lens. A returning light beam reflected by the optical disc is split from the light beam emitted by the semiconductor laser and guided to the light detector by the optical system.
The position of the objective lens in the direction of its optical axis is adjusted by an actuator which will be further discussed later so that the light beam emitted from the semiconductor laser follows displacements of the optical disc in a direction orthogonal to the optical disc surface caused by warping and the like of the optical disc and is always focused on the recording surface of the optical disc. At the same time, the position of the objective lens in a direction orthogonal to the optical axis of the objective lens is adjusted by the actuator so that the position of a spot formed on the optical disc by the light beam emitted from the semiconductor laser follows eccentricity of the optical disc and snaking of a track formed on the optical disc.
This adjustment of the focus position and the spot position of the light beam emitted from the semiconductor laser on the recording surface of the optical disc is carried out by adjusting the position of the objective lens in the optical axis direction of the objective lens and the position of the objective lens in the direction perpendicular to this optical axis. An electromagnetically driven actuator is used for this positional adjustment of the objective lens.
This actuator is generally called an objective lens actuator or a biaxial actuator, and typically comprises a lens holder in which the objective lens is mounted, a plurality of elastic supporting members, and a driving part for creating driving forces. The lens holder is supported on a mounting part by the plurality of elastic supporting members so that the position of the objective lens in the direction of the optical axis thereof, that is, the focus position, and the position of the objective lens in a direction orthogonal to the optical axis, that is, the tracking position, are adjustable. An example of this kind of biaxial actuator Will be described below with reference to FIG. 1.
The biaxial actuator such as being described above is constructed for example as shown in FIG. 1. In FIG. 1, a biaxial actuator 1 has a lens holder 2 in a front end of which is mounted an objective lens 2a, and a coil bobbin 3 attached by means of adhesive or the like to this lens holder 2.
The lens holder 2 is supported movably with respect to a mounting part 4 in two directions perpendicular to each other, that is, a tracking direction shown by the mark Trk and a focusing direction shown by the mark Fcs, by two pairs of elastic supporting members 5 each having one end fixed to a side of this lens holder 2 and the other end fixed to a mounting part 4.
The coil bobbin 3 is constructed as shown schematically in FIG. 2. That is, the coil bobbin 3 has an opening 3a passing therethrough in the focusing direction and has a focusing coil 3b wound around this opening 3a and two tracking coils 3c located at left and right on the front side of this focusing coil 3b.
By introducing currents into the coils 3b and 3c, magnetic flux producedby the coils 3b and 3c interacts with magnetic flux from an inner yoke 6a and a facing yoke 6b of a yoke 6 mounted on the mounting part 4 and magnets 7a and 7b mounted on the inner yoke 6a and the facing yoke 6b. Currents flowing through the inner parts of the tracking coils 3c extending vertically to the paper surface create a force causing the lens holder 2 to move in the tracking direction Trk according to Flemings left hand rule. Hence, only the inner vertical parts (3d, 3d in FIG. 5) of the tracking coils 3c are effective for the tracking, and the horizontal parts and the outer vertical parts 3e of the tracking coils 3c do not contribute to tracking and are ineffective.
The elastic supporting members 5 are made of an elastic material and are fixed between the lens holder 2 and the mounting part 4 in parallel with each other. Each of the elastic supporting members 5 has a substantially rectangular end area 5a on the mounting part 4 side. The end areas 5a each have slits formed on both sides of a long, thin main portion extending in the front/rear direction of the respective elastic supporting member 5. A viscous substance is coated onto these end areas 5a so as to cover the main portions and the slits and is hardened. This viscous substance is for example an ultraviolet light hardening type viscous substance, and, after being coated, is hardened by being irradiated with ultraviolet light. As a result, the hardened viscous substance acts as a vibration-controlling damper and suppresses vibration of the elastic supporting members 5.
In the biaxial actuator 1 thus constructed, driving currents are supplied to the coils from outside and flux produced by the coils interacts with flux from the yoke 6 and the magnets 7 and the coil bobbin 3 is moved in the tracking direction Trk and the focusing direction Fcs. In this way, the objective lens 2a attached to the lens holder 2 is moved in the focusing direction and the tracking direction.
When the lens holder 2 is thus moved in the focusing direction and the tracking direction, the lens holder 2 tends to vibrate in the directions of its movement, but the vibration is suppressed by the damping action of the viscous substance provided on the rear ends of the elastic supporting members 5. In this way, the lens holder 2 is stopped in a stable state at a predetermined position.
The above-mentioned yoke 6 is substantially U-shaped in cross-section, and a Gap between the upper ends of the inner yoke 6a and the facing yoke 6b is open. To close off this open end of the yoke 6, a yoke bridge 6c is attached to the upper ends of the inner yoke 6a and the facing yoke 6b with an adhesive. The yoke bridge 6c thus increases the sensitivity of the magnetic circuit formed by the yoke 6 and the magnets 7a and 7b and prevents leakage of flux therefrom, and also acts as a stopper against movement of the coil bobbin 3 in the focusing direction.
When as shown in FIG. 2 two magnets 7a and 7b are attached to the inner surfaces of the inner yoke 6a and the facing yoke 6b of the above-mentioned yoke 6, a high flux density is obtained in the magnetic circuit formed by the magnets 7a and 7b and the yoke 6, but there has been the problem that because two magnets 7a and 7b are being used the overall biaxial actuator 1 is heavy. To overcome this problem it is possible to adopt a measure for which one of the magnets is eliminated.
For example, FIG. 3 shows a case wherein a magnet 7 is mounted on only the inner surface of the facing yoke 6b of the yoke 6.
With this arrangement, because only one magnet 7 is being used, the weight is reduced; however, points of action of the driving forces acting on the coils during focusing and tracking move away from the heavy objective lens 2a. Consequently, to keep these points of action usually around the center of gravity of the movable part, it is necessary to provide a balance such as a weight on the side opposite the objective lens and make the rear part of the movable part of the biaxial actuator 1 heavy. Because of this there has been the problem that the complicated work of mounting a balance is made necessary and furthermore the biaxial actuator 1 as a whole becomes heavy and large.
On the other hand, FIG. 4 shows a case wherein a magnet 7 is mounted on only the inner surface of the inner yoke 6a of the yoke 6. In this construction the weight decreases because only one magnet 7 is being used, and also the points of action of the driving force acting on the coils during focusing and tracking move toward the objective lens 2a. Therefore, to bring the center of gravity of the movable part to these points of action, it is only necessary to make the rear part of the movable part of the biaxial actuator 1 lighter and the whole actuator can thus be made smaller.
However, in a biaxial actuator 1 with the construction shown in FIG. 4, a current I flows in mutually opposite directions in the tracking coils 3c such as shown in FIG. 5. That is, in FIG. 5, a current flows clockwise in the tracking coil 3c on the left side and a current flows counterclockwise in the tracking coil 3c on the right side.
Therefore, the direction of the flux in the magnetic circuit of the yoke 6 and the magnet 7, such as shown in FIG. 6, is from the inner yoke 6a toward the facing yoke 6b and passes through the tracking coils 3c. As a result, because the currents in the inner vertical parts 3d of the tracking coils 3c extending vertically are flowing toward the rear of the paper surface, according to Fleming's left hand rule, driving forces F1 and F2 are respectively produced in the tracking direction as shown in FIG. 6.
In this case, because the width of the magnet 7 is larger than the distance between the centers of the tracking coils 3c, flux from around the both ends of this magnet 7 leaks toward the outer sides as shown by arrows toward outer side M1 and M2 and leakage flux is incident diagonally toward the outer side vertical parts 3e of the tracking coils 3c.
Because current flows through these outer side vertical parts 3e of the tracking coils 3c so as to come out from the paper surface, according to Fleming's left hand rule, driving forces F3, F4 are produced by leakage flux as shown in FIG. 6. Since the directions of these driving forces F3 and F4 are mutually different, as shown in FIG. 7, a moment M3 is produced by the driving forces F3 and F4 on the movable part of the biaxial actuator 1, i.e. the movable part made up of the objective lens 2a, the lens holder 2 and the coil bobbin 3, and consequently the lens holder 2 is turned. As a result, there has been the problem that it becomes impossible to carry out accurate tracking.
Also, in the biaxial actuator 1 thus constructed, because the yoke bridge 6c is attached to the open end of the yoke 6 with adhesive, there has been the problem that the adhesion strength is relatively low at about 300 to 500 g and furthermore, with respect to temperature changes, the strength decreases greatly with a temperature increase of about 35.degree. C.
In view of the points described above it is an object of the invention to provide a biaxial actuator the whole of which is constructed to be small and light weighted and with which accurate tracking can be carried out and also preferably in which the attachment strength of a yoke bridge is improved.